Field of the Invention
The invention relates to a magnetic-inductive flowmeter for measuring flow of a flowing medium, having a measuring tube, having a magnetic field generator for generating a magnetic field at least partially interfusing the measuring tube, and having at least one measuring electrode for tapping a measuring voltage induced in the flowing medium, wherein the measuring tube has a central measuring section including the measuring electrodes that is flat on one side having a planar measuring tube portion, called portion in the following, and wherein at least one coil core and one pole shoe belong to the magnetic field generator.
Description of Related Art
According to Faraday's law of induction, an electric field strength is formed perpendicular to the direction of flow of the medium and perpendicular to the magnetic field in a flowing, electrically conductive medium interfused by a magnetic field. Faraday's law of induction is thus exploited in magnetic-inductive flowmeters in that a magnetic field usually fluctuating over time during the measurement process is generated by means of a magnetic field generator usually having at least one magnetic field coil, and that the magnetic field at least partially interfuses the electrically conductive medium flowing through the measuring tube. Here, the generated magnetic field has at least one component perpendicular to the longitudinal axis of the measuring tube or perpendicular to the direction of flow of the medium.
If the magnetic-inductive flowmeter being discussed here has at least one magnetic field generator “for generating a magnetic field running perpendicular to the longitudinal axis of the measuring tube”, then the magnetic field preferably runs perpendicular to the longitudinal axis of the measuring tube or perpendicular to the direction of flow of the medium, however, it is sufficient when a component of the magnetic field runs perpendicular to the longitudinal axis of the measuring tube or perpendicular to the direction of flow of the medium.
It is also described above that the magnetic field generator is used for generating a preferably alternating magnetic field. This expresses that it is not of importance for the teaching of the invention that this is an alternating magnetic field, i.e., an alternating electromagnetic field. However, it is to be pointed out that magnetic-inductive flowmeters predominantly have magnetic field generators for generating alternating magnetic fields.
It is also described above that the magnetic-inductive flowmeter being discussed here also has at least one measuring electrode for tapping a measuring voltage induced in a flowing medium, often, two electrodes are present. Preferably, the measuring electrodes come into contact with the medium. Preferably, the virtual connection line of the two measuring electrodes runs at least essentially perpendicular to the direction of the magnetic field interfusing the measuring tube perpendicular to the longitudinal axis of the measuring tube. In particular, the measuring electrodes can be provided in such a manner that their virtual connection line actually runs—more or less—perpendicular to the direction of the magnetic field interfusing the measuring tube.
It has already been described above that the measuring electrodes are, in particular, such that they come into contact with the medium. Indeed, of course, the electric field strength generated by induction in the flowing, electrically conductive medium can be tapped by measuring electrodes having direct, i.e., galvanic contact with the medium as a measuring voltage. However, there are magnetic-inductive flowmeters in which the measuring voltage is not tapped by measuring electrodes having direct, i.e., galvanic contact with the medium, rather the measuring voltage is capacitively determined.
Magnetic-inductive flowmeters known from the prior art in German Patent DE 692 32 633 C2, DE 199 07 864 A1 and corresponding U.S. Pat. No. 6,453,754 B1, German Patent DE 100 64 738 B4 and corresponding U.S. Pat. No. 6,564,612 B2, DE 102 43 748 A1 and corresponding U.S. Pat. No. 6,804,613 B2, German Patent Application DE 10 2008 005 258 A1 and corresponding U.S. Pat. No. 7,971,493 B2 and German Patent Application DE 10 2011 112 703 A1 and corresponding U.S. Patent Application 2012/0066301 A1 as well as European Patent Applications EP 0 704 682 A1 and EP 0 834 057 A1 and corresponding U.S. Pat. No. 6,092,428 are referred to as examples. Reference is made, in particular, to German Patent Application DE 10 2008 057 756 A1 and corresponding U.S. Pat. No. 8,286,502 B2, from which the magnetic-inductive flowmeter described in the introduction is known. The measuring tube in this known magnetic-inductive flowmeter has a cross-section that changes over its length and the cross-section in the central section of the measuring tube, called measuring section above, is less than at the beginning of the measuring tube and the end of the measuring tube. Thereby, the cross-section of the measuring tube in its central section, i.e., in the measuring section, is rectangular, or optionally square.
On the other hand, the invention is based on a magnetic-inductive flowmeter, in which the measuring section of the measuring tube necessarily has only a planar measuring tube portion, called portion above. Namely, the measuring section in magnetic-inductive flowmeters according to the invention can have two or more planar portions, it only being important for the following teaching of the invention that one planar measuring tube portion is sufficient.
The known magnetic-inductive flowmeters are often a “sturdy construction” in that the measuring tube and/or the measuring device housing is formed of metal. As a general rule, these measuring tubes are tubes, i.e., cylindrical hollow bodies having a circular cross section. The measuring device housings are also often designed as cylindrical hollow bodies with a circular cross section or an essentially circular cross section. Further, it holds true for most known magnetic-inductive flowmeters that the measuring device housings have end flanges and connection flanges formed of metal on both sides. On the one hand, these flanges, with which the two ends of the measuring tube are—directly or indirectly—connected, terminate the flowmeter, leading to the term “end flange”. On the other hand, the flanges are used for connection of both sides of the flowmeter to the corresponding piping flanges, thus “connection flange”.
Among many other things and above all, magnetic-inductive flowmeters have to meet substantial requirements for measuring accuracy. It is to be taken into consideration here that the measuring voltage induced in the flowing, electrically conductive medium is relatively low. This holds true for “normal conditions”. “Normal conditions” are understood as a flow velocity that is not particularly low and a conductivity of the flowing medium that is not particularly low. In the case of “complicated conditions” in terms of measuring, i.e., low flow velocity and/or low electric conductivity of the flowing medium, particularly low measuring voltages are induced, with the consequence that relatively low—in absolute terms—interfering voltages significantly influence the measuring accuracy.
The result hereby is that a strong as possible magnetic field should be generated in the measuring tube so that, preferably, also all attenuations are reduced as much as possible. This implies that, e.g., in the prior art, the wall thickness in the measuring section is chosen to be as small as possible.
In an implementation of the magnetic-inductive flowmeter known from German Patent Application DE 10 2008 057 756 A1 and corresponding U.S. Pat. No. 8,286,502 B2, the wall thickness of the measuring tube in the measuring section is less than at the beginning and the end of the measuring tube. Due to the capacity of the measuring tube to withstand pressure, at least one reinforcement connecting the measuring tube to the measuring device housing is provided in the area of the measuring section.
It is stated above that known magnetic-inductive flowmeters are often overall of a “sturdy construction” in that the measuring tube and/or the measuring device housing consist/s of metal. Increased stability can be necessary—even with a metallic measuring tube—when a high pressure load exists. However, it can be appropriate to produce the measuring tube and optionally also the measuring device housing of a material that is relatively less durable—and thus less expensive—, in particular of relatively inexpensive plastic.
The necessity for an increased stability can thus result, on the one hand, due to the use—i.e., the prevailing pressure. On the other hand, this may be due to the choice of material—e.g., plastic.
This goes hand in hand with another conflict of interests: On the one hand, the measuring tube, even in the area of the measuring section, has to have a considerable thickness in a magnetic-inductive flowmeter, whose measuring tube is formed of a relatively less durable plastic. Such a thickness is also then necessary when the measuring tube is formed of metal, but high pressure is able to arise in the measuring tube. On the other hand, the magnetic field generator should generate a relatively strong and extensively homogenous magnetic field in the measuring tube. In the prior art, for example, this leads to the measuring tube being implemented with a measuring section that is flat on one side with a planar measuring tube portion, in the following always just called portion, and the measuring tube only has a relatively thin thickness in the area of the portion.
What is described above leads to the magnetic field generator being able to generate a relatively strong and extensively homogenous magnetic field in the measuring tube because the pole shoe can be arranged directly and flat against the planar portion and thus close to the inside of the measuring tube.